Field of the Invention
The present invention is related to the field of pelletizers for production of pellets from polymer containing materials and other pelletizable formulations.
Description of the Related Art
Various pelletizer designs are disclosed in U.S. Pat. No. 5,059,103 (“the '103 patent”), U.S. Pat. No. 7,033,152 (“the '152 patent”), and U.S. Pat. No. 8,303,871 (“the '871 patent”), all of which patents are co-owned by the applicant and assignee of the instant application. The disclosures of the '103 patent, the '152 patent and the '871 patent are relevant to the subject matter of the instant application and therefore these three patents are hereby expressly incorporated herein by reference as if fully set forth in the instant application.
The prior art pelletizer design shown in FIGS. 1, 5 and 5A utilizes a pelletizer shaft 100 that is bolted to the motor shaft 105 of motor 190 with set screws 110. A cutter hub holder 115 is retained on the pelletizer shaft 100 with a bolt 120 and has a spring 125 for adjustment as the blades 130 wear down. This cutter hub holder 115 has multiple keys 135, typically 2 to 4 keys are used, which are welded in place to slide in key slots that are cut in the pelletizer shaft 100. Several problems with this design may arise.
First, the keys 135 can sometimes break loose from the weld that holds them in the cutter hub holder 115. Further, due to the nature of welding, the keys may not be precisely positioned to exactly match the key slots in the pelletizer shaft, resulting in less than the desired amount of surface contact between each of the keys and their corresponding slot. As a result, torque that is transmitted from the cutter hub to the pelletizer shaft during operation of the pelletizer may be borne by less than all of the keys, creating even higher stresses on the torque-bearing keys which can impair pelletizer performance and/or lead to breakage of the keys.
Second, the keys can wear into the sides of the key slots in the pelletizer shaft 100. Since they do not extend beyond the end of the pelletizer shaft 100, the keys can become caught within notches formed by the wear of the keys against the sides of the key slots. The wear pattern of these notches creates a “hook” that can trap the keys and prevent the spring 125 from pushing the cutter hub holder 115 forward to compensate for blade wear, causing poor cutting performance and thus poor pellet quality.
Third, fines that are generated when cutting the polymer strands with the blades can accumulate and pack within the key slots, also locking the cutter hub holder 115 in place and preventing movement thereof as the blades 130 wear down or when new blades are installed. Fines packed in the key slots can also produce cross contamination from one pelletizing run to the next.
Another prior art pelletizer design is shown in FIGS. 2, 6 and 6A and utilizes a pelletizer shaft 200 that is bolted to the motor shaft 205 of motor 290 with set screws 210. A cutter hub holder 215 is retained on the pelletizer shaft 200 by threaded engagement with the motion rod 220 which provides for adjustment of cutter hub position as the blades 225 wear down.
Like the design shown in FIG. 1, the FIG. 2 cutter hub holder 215 has multiple keys 230 welded thereto that can sometimes break loose from the weld holding them in the cutter hub holder 215. These keys can also wear into the sides of the key slots in the pelletizer shaft 200 since they do not extend out of the end of the pelletizer shaft 200, again creating the “hook” wear pattern that requires more effort to push the cutter hub holder 215 forward to compensate for blade 225 wear and causing poor cutting performance and thus poor pellet quality. Also, like the FIG. 1 design, fines that are generated from trying to cut the polymer strands by the blades can accumulate and pack within the key slots, preventing movement of the cutter hub holder 215.
An additional problem with both the FIG. 1 and FIG. 2 designs is that the cutting area within the cutting chamber or water box can become agglomerated with molten polymer or polymer blend material, allowing the polymer to wrap around the cutter hub and blades and flow either up the slurry outlet piping 150, 245 or down the water or fluid inlet piping 145, 240, or into both the inlet and the outlet. Simultaneously, the molten polymer can begin to quickly solidify before the operator can safely get to the pelletizer and unclamp the pelletizer from the cutting chamber or water box. The pelletizer, now being held in position by the solidifying polymer, cannot be easily removed from the cutting chamber 155, 250. As used herein, “polymer” is intended to refer to both pure polymer melts as well as extrudable material that contains a mixture of one or more polymers and other non-polymer material such as biomaterials, additives, and the like. Also, the terms “water box” and “cutting chamber” are used interchangeably herein.
There is no easy way with either the FIG. 1 or the FIG. 2 design to remove the cutter hub 140, 235 or the cutter hub holder 115, 215 from the pelletizer shaft 100, 200 so the operator can remove the pelletizer and get into the cutting chamber to remove the agglomerated polymer and clear the cutter blades of any obstructions so to be able to re-start the pelletizer and process line. Ideally, in the case of polymer agglomeration the operator should proceed to take certain disassembly steps such as unbolting the cutting chamber 155, 250 and die plate 160, 255 from the upstream equipment to clean out the polymer wrap. Alternatively, in the FIG. 2 design, the operator can remove the fan cover (not shown) on the back of the motor 290, disconnect the motion rod 220 from the motion rod adjuster (not shown) and then unscrew the motion rod 220 from the cutter hub holder 215. If the motion rod is not disconnected from the correct point, however, then the forward travel must be readjusted when reassembling the components. If the forward travel is not adjusted correctly, the operator could, in the best case, not get full blade life out of the blades 225, or in the worst case, the operator could run the cutter hub 235 into the face of the die 255 as the blades 225 wear down, which will damage the face of the die 255 requiring that the die be replaced or repaired.
Unfortunately, rather than undertaking disassembly procedures such as those just described, many operators will try to introduce pry bars to increase leverage to get the pelletizer separated from the cutting chamber. The heavy duty pry bars can damage the precision mating surfaces of the pelletizer front-end and the connecting flange of the cutting chamber. Further, the pry bars put very high stresses on the various components on the front end and throughout the pelletizer and its motor. In some extreme cases, operators or maintenance staff have brought in a forklift truck to pull the pelletizer away from the cutting chamber. Needless to say, such forceful separation can severely damage the pelletizer and its components, creating costly failures, extensive down time awaiting replacement parts to repair the damage, and/or safety hazards.
Hence, there is a need for a cutter hub drive mechanism that produces better torque transmission from the pelletizer shaft to the cutter hub while reducing the likelihood of drive mechanism breakage and drive key entrapment arising from wear patterns formed in the pelletizer shaft by the keys. In addition, there is a need for a structure to prevent the entry of fines into the drive key area, and also a process by which the removal of agglomerated polymer within the cutting chamber is shortened and simplified.